Conjugated dienes such as 1,3-butadiene and isoprene undergo a variety of catalytic oligomerization reactions to give cyclic or acyclic oligomers. These oligomers are valuable feedstocks in the industrial production of fine organic chemicals. For example, the dimers and trimers are utilized as intermediates for synthesizing flame retardants, terpenoid and sesquiterpenoid compounds of biological interest, and fragrances.
Various coordination catalyst systems based on nickel, palladium, cobalt, titanium, chromium, and iron have been reported in the prior art for catalyzing the oligomerization of conjugated dienes (see, e.g., R. Baker, in Chemical Reviews 1973, Volume 73, Page 487). The majority of these catalyst systems, however, have no practical utility because they have low activity and poor selectivity. The resulting oligomerization product is often a complicated mixture of cyclic and acyclic dimers, trimers, tetramers, and higher oligomers. Furthermore, some oligomerization catalyst systems also generate a certain amount of polymer in the oligomerization product mixtures.
Several iron-based coordination catalyst systems are known in the prior art for the oligomerization of conjugated dienes. The Bulletin of Chemical Society of Japan 1965, Volume 38, Page 1243 discloses a process for the oligomerization of 1,3-butadiene by using a catalyst system comprising iron(III) acetylacetonate and triethylaluminum. The Bulletin of Chemical Society of Japan 1966, Volume 39, Page 1357 discloses a process for the oligomerization of 1,3-butadiene by using a catalyst system comprising iron(III) acetylacetonate, triethylaluminum, and triphenylphosphine. And, the Journal of Organic Chemistry 1965, Volume 30, Page 1661 discloses a process for oligomerizing 1,3-butadiene in the presence of a catalyst system comprising iron(III) chloride, triphenylphosphine, and triethylaluminum. All of these iron-based catalyst systems, however, have very low activity and poor selectivity, and the resulting oligomerization product is a mixture of cyclic and acyclic dimers, trimers, and higher oligomers, as well as polymer. Therefore, these iron-based catalyst systems have no industrial utility.
Because the oligomers of conjugated dienes are useful and the catalyst systems known heretofore in the art have many shortcomings, it would be advantageous to develop a new and significantly improved catalyst system that has high activity and selectivity for oligomerizing conjugated dienes into oligomers.